Valves are used in a variety of industries to control the flow of fluids. In particular, gate valves are used extensively in the oil and gas industry to control the flow of produced fluids at various stages of drilling or production. Most gate valves used in this industry comprise a valve body having a longitudinal flow bore and a transverse gate cavity that intersects the flow bore. A gate having an opening extending transversely therethrough is disposed in the gate cavity. A valve stem is provided for moving the gate between an open position, in which the gate opening is aligned with the flow bore, and a closed position, in which the gate opening is offset from the flow bore. The gate cavity of the valve body is covered by a bonnet having an axial bore through which passes the valve stem.
Such a gate valve is associated to a valve operator assembly for selectively driving the valve stem up and down in order to close and open the gate valve. A valve operator assembly generally comprises an input member to exert a rotational motion by a manual actuation by a hand-wheel or an electric or pneumatic or hydraulic actuation. The valve operator assembly further comprises a transmission mechanism to convert a rotational motion into axial motion of the valve stem. To quickly open and close the gate valve with a minimum number of turns, the transmission mechanism may be a screw mechanism, such as a roller screw mechanism, a ball screw mechanism or an acme screw with or without gear reducer, in order to reduce the operating torque.